Global Warming Would Foster Spread of Dengue Fever into Some Temperate Regions

Scientists using computers to simulate the general circulation of the earth's climate have predicted that rising global temperatures will increase the range of a mosquito that transmits the dengue fever virus. Dengue fever is now considered the most serious viral infection transmitted in man by insects, whether measured in terms of the number of human infections or the number of deaths.

Most of the new areas of mosquito encroachment were predicted to be temperate regions that currently border on endemic zones. These fringe areas represent places where humans and the primary carrier, the mosquito Aedes aegypti, often co-exist, but where lower temperatures now limit disease transmission. Lead author Jonathan Patz, M.D., M.P.H., from the Johns Hopkins School of Public Health said, "Since inhabitants of these border regions would lack immunity from past exposures, dengue fever transmission among these new populations could be extensive." Unlike the yellow fever virus, carried by the same mosquito, the dengue virus is not vulnerable to any vaccine or drug.

The research was published in the March issue of Environmental Health Perspectives, the monthly journal of the National Institute of Environmental Health Sciences.

The only serious outbreak of dengue fever in the continental United States occurred in Philadelphia in 1780, when the ship-borne introduction of the virus apparently coincided with an unusually hot summer.

The researchers used three different general circulation models to predict the patterns of global climate change; all three showed that dengue's epidemic potential increases with a relatively small temperature rise. The higher a virus's epidemic potential, the fewer mosquitoes are necessary to maintain or spread dengue in a vulnerable population.

The geographic range of Ae. aegypti is limited by freezing temperatures that kill overwintering larvae and eggs, so that dengue virus transmission is limited to tropical and subtropical regions. Global warming would not only increase the range of the mosquito but would also reduce the size of Ae. aegypti's larva and, ultimately, adult size. Since smaller adults must feed more frequently to develop their eggs, warmer temperatures would boost the incidence of double feeding and increase the chance of transmission.

In addition, the time the virus must spend incubating inside the mosquito is shortened at higher temperatures. For example, the incubation period of the dengue type-2 virus lasts 12 days at 30 C, but only seven days at 32-35 C. Shortening the incubation period by five days can mean a potential three-fold higher transmission rate of disease.

An estimated 2.5 billion people are currently at risk from dengue infection, and since the late 1970s dengue has reemerged in the Americas, with 280,000 reported cases reported in Latin America in 1995 alone. Outbreaks in urban areas infested with Ae. Aegypti, can be explosive, involving up to 70-80% of a population.

While the accuracy of long-term climate forecasting by computers will continue to be questioned, the global warming scenarios predicted by the various different computer models are increasingly coming to resemble one another. Climatologists are projecting that global climate will change at an unprecedented rate over the next century. Dr. Patz said, "Our study makes no claim that climate factors are the most important determinants of dengue fever. However, our computer models illustrate that climate change may have a substantial global impact on the spread of dengue fever."

The study was funded in part by the Climate Policy and Assessment Division of the U.S. Environmental Protection Agency, the National Institute of Public Health and Environment (RIVM) (the Netherlands), and the Center for Medical, Agricultural, and Veterinary Entomology of the U.S. Department of Agriculture.

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